Submarine fresh groundwater discharge estimation in the intertidal zone based on dynamic salinity simulation

Currently, limited studies focused on the variations in submarine fresh groundwater discharge rates, seawater recirculation rates, and their respective proportions with tidal level change in sandy intertidal zones. This study conducted in-situ dynamic monitoring of the salinity at the groundwater seepage face in a sandy tidal flat in Xiamen Bay. Using variable-density flow numerical model, the dynamic groundwater salinity in the intertidal zone was simulated to calculate the submarine fresh groundwater discharge in the intertidal zone. The results show that groundwater salinity at different locations in the intertidal zone exhibits varying patterns with the tides. The submarine groundwater discharge at a typical profile was estimated to be 17.47 m³/(m·d), in which the submarine fresh groundwater discharge was calculated to be 3.19 m³/(m·d), accounting for 18.26% of the total submarine groundwater discharge. In contrast, recirculated submarine groundwater discharge was estimated to be 14.28 m³/(m·d), constituting 81.74% of the total submarine groundwater discharge and thus playing a dominant role. Under the influence of tides, seawater infiltration and submarine groundwater discharge are alternate. The alternation during ebb tide occurs slightly earlier than the slack water level, while during flood tide, it occurs slightly later. The peak moment of seawater infiltration rate and submarine groundwater discharge rate precede the tidal peaks at high and low tides, respectively. Throughout the tidal cycle (from high tide to the next high tide), the proportion of submarine fresh groundwater discharge gradually increases. The variation in groundwater salinity in the intertidal zone can represent the changes in submarine fresh groundwater discharge. The proportion of submarine fresh groundwater discharge in the total submarine groundwater discharge is relatively high in this typical profile than that in the previous studies. It indicates the presence of a substantial amount of submarine fresh groundwater discharge in the intertidal zone. Through sensitivity analysis, it reveals that inland hydraulic head has significant influences on submarine fresh groundwater discharge and spatial distribution of salinity. This study can provide scientific information for the development and utilization of coastal groundwater and the management of coastal environments.